This invention relates to the production of uniform spheroidal beads and more particularly to apparatus and a method for producing uniform spherical droplets of materials which subsequently form solid, uniform beads.
The bead forming material is typically a hydrosol, or slurry containing silica, alumina, or silica-alumina. The beads are typically metal oxides which are used as a catalyst, a catalyst support, or matrix, an adsorbent, or an ion exchange material.
Spheroidal beads offer distinct advantages when employed in fixed bed reactor applications; for example, improved packing efficiencies and increased attrition resistance are realized.
The physical and structural properties of a catalyst significantly influence its activity and durability. More particularly, the physical and structural properties of the catalyst support or base material significantly influence activity and durability. The pore structure including size distribution and volume determines the extent and accessibility of surface area available for contact of the catalytic material and the reactants. Catalytic activity often depends on the rate of diffusion of reactants and products in and out of the interstices of a catalyst. Increased pore size may facilitate the diffusion of reactants and reaction products, but catalytic activity is also a function of surface area and packing density.
Spheroidal catalyst beads have many advantages over other shapes because they permit uniform packing so that variations in pressure drop are minimized and the tendency of a reactant stream to channel through the bed out of effective contact with the catalyst is reduced. Better flow properties, in the case of a moving bed system, are produced.
The foregoing and other desirable catalyst properties are discussed in U.S. Pat. No. 4,318,896--Schoenover which also discusses the five general methods of preparing spheroidal particles of a size suitable for commercial operation. Of these, the spray drying method and the method of dropping particles into an oil bath are widely used. In these methods, drops of a catalyst forming liquid are produced and allowed to harden. The hardening takes place in a stream of air in the spray drying technique and in a water immiscible liquid such as oil in the other technique.
Processes for manufacturing alumina particles are shown, for example, in U.S. Pat. Nos. 3,558,508, 4,179,408 and 4,318,896. Processes for manufacturing silica particles are shown in U.S. Pat. No. 3,872,217 and processes for manufacturing silica-alumina particles as shown for example, in U.S. Pat. No. 3,986,978.
It has long been known to prepare catalyst supports, especially spheroidal alumina particles, by the well-known oil-drop method, described by Hoekstra in U.S. Pat. No. 2,620,314. Briefly, the method comprises commingling an acidic alumina hydrosol with a gelling agent characterized as a weak base which hydrolyzes to ammonia with increasing temperature, and dispersing the mixtures as droplets in a hot oil bath generally contained in a vertical column or forming tower. The forming oil is typically a light or heavy gas oil chosen principally for its high interfacial tension with respect to water. Thus, as each droplet penetrates the oil surface, it draws into a spherical shape. The droplets are principally water at this stage and, being insoluble in the oil, they tend to assume a shape having the least surface area for its volume. A second effect is that the formed hydrosol droplets gravitating to the bottom of the forming oil are progressively gelled to a stage sufficient to maintain the structural integrity of the resulting hydrogel spheres during the subsequent processing thereof. In any case, the formed hydrogel spheres are subsequently aged, usually in the hot forming oil and thereafter washed, dried and calcined, usually in an oxidizing atmosphere at 425.degree. to 750.degree. C.
The above-described method basically uses an internal gelling agent, such as hexamethylenetetramine, that is added to the feed before drop formation and that releases ammonia in the hot oil bath. U.S. Pat. No. 3,558,508 to Keith et al. describes an oil-drop method employing an external gelation technique in which gaseous ammonia is introduced into the bottom of a column containing the water-immiscible liquid and coagulates the droplets by contacting their external surfaces. The Keith et al. process is based to a considerable extent on the use of specific alumina feed prepared by acidic hydrolysis of finely divided aluminum.
Spherical alumina particles may also be formed by the hydrocarbon/ammonia process described in Olechowska et al., "Preparation of Spherically Shaped Alumina Oxide", INTERNATIONAL CHEMICAL ENGINEERING, Volume 14, No. 1, pages 90-93, January, 1974. In this process, droplets of a slurry of nitric acid and dehydrated aluminum hydroxide fall through air into a column containing hydrocarbon and ammonia phases. The droplets assume spheroidal shapes in passing through the water-immiscible liquid and then are coagulated to firm spheriodal beads or pellets in the coagulating medium. Similar processes utilizing pseudosol feeds and hydrochloric acid are described in:
1. Katsobashvili et al., "Formation of Spherical Alumina and Aluminum Oxide Catalysts by the Hydrocarbon-Ammonia Process--1. The Role of Electrolytes in the Formation Process", KOLLOINDNYI ZHURNAL. Vol. 28, No. 1, pp. 46-50, January-February, 1966;
2. Katsobashvili et al, "Preparation of Mechanically Strong Alumina and Aluminum Oxide Catalysts in the form of Spherical Granules by the Hydrocarbon-Ammonia Forming Method", ZHURNAL PRIKLADNOI KHIMII, Vol. 39, No. 11, pp. 2424-2429, November 1966; and
3. Katsobashvili et al., "Formation of Spherical Alumina and Aluminum Oxide Catalysts by the Hydrocarbon-Ammonia Process-Coagulational Structure Formation During the Forming Process", KOLLOINDNYI ZHURNAL, Vol. 29, No. 4 pp. 503-508, July-August, 1967.
Subsequent aging of the catalyst droplets in an aqueous bath may lead to stress cracks or fractures, because of different salt concentrations in the bath. The problems of stress cracks or fractures due to salt concentration gradients, and one method solution of these problems, is disclosed in U.S. Pat. No. 4,250,058.
Whenever catalysts are transferred from an oil phase to an aqueous phase, there is an interface which tends to accumulate any dirt which may be present in the system and hold up some particles of catalyst.
It is possible to minimize the problems of getting catalyst particles through the interface by adding a surfactant or detergent to the water phase, but this represents added cost and added complications.
The spheroidal beads produced by the prior art are not as uniform as is desired. Uniformity of bead size enhances uniform packing which is desirable for the reasons previously discussed. Further, the beads produced by prior art techniques are often not as large as desired.
It is an object of the present invention to produce uniform droplets which harden into uniform size spheroidal beads in either the spray drying or oil bath hardening technique.